1. Field of the Invention
The present invention relates to a ceramic heater to be used in a glow plug for preheating a diesel engine or in a like device, to a method for manufacturing the same, and to a glow plug using the same.
2. Description of the Related Art
A conventionally known ceramic heater for the above-mentioned applications is configured such that a resistance-heating member formed of an electrically conductive ceramic is embedded in an insulating ceramic substrate. In such a ceramic heater, electricity is supplied to the resistance-heating member via metallic leads formed of tungsten or a like metal. However, use of the metallic leads involves a corresponding increase in the number of components, possibly resulting in an increase in the number of manufacturing steps and thus an increase in cost. In order to cope with the problem, Japanese Patent No. 3044632 discloses an all-ceramic-type heater structure in which (1) a first resistor portion serves as a major resistance-heating portion, and (2) a second resistor portion, which is formed of an electrically conductive ceramic having electrical resistivity lower than that used to form the first resistor portion, serves as an electricity conduction path to the first resistor portion. This structure eliminates the use of metallic leads.
The integration of resistor portions that have different electrical resistivities facilitates implementation of a ceramic heater having a so-called self-saturation-type heat generation characteristic. These ceramic heaters function in the following manner. At an initial stage of electricity supply, a large current flows to the first resistor portion via the second resistor portion, thereby promptly increasing the temperature. And when the temperature rises to be near a target temperature, the current flow is controlled by means of an increase in electric resistance of the second resistor portion. Japanese Patent Application Laid-Open (kokai) No. 2000-130754 also discloses this effect as well as a ceramic heater structure in which electricity is supplied, via metallic leads, to a ceramic resistor configured such that two resistor portions of different electrical resistivities are joined together.
In the above-described ceramic heaters, a joint interface between ceramic resistors formed of different materials is inevitably formed. Usually, electrically conductive ceramics that have different electrical resistivities also have considerably different coefficients of linear expansion. Accordingly, in an application involving frequent repetition of temperature rise and cooling (as in the case of a glow plug), thermal stress induced by the above-mentioned difference in coefficient of linear expansion tends to concentrate at the joint interface between the different resistor portions. Particularly, in the case of the structure disclosed in Japanese Patent No. 3044632, in which the resistor portions are joined via a flat interface that is perpendicular to the axis, the area of the joint interface is small, and thus the above-described stress concentration is likely to fracture the resistor along the joint interface. To cope with this drawback, Japanese Patent Application Laid-Open (kokai) No. 2000-130754 proposes a structure in which a circular recess is formed on an end part of the first resistor portion, and a protrusion is formed on an end part of the second resistor portion so as to be fitted into the recess, thereby increasing the area of the joint interface and thus enhancing the strength of the joint.
Although the conventional ceramic heaters are generally thought to be acceptable, they are not without shortcomings. These shortcomings include the following. (1) Since the protrusion and the recess must be formed independently on the corresponding joint interfaces, when the resistor is to be formed through injection molding and firing, the two resistor portions must be formed independently of each other by use of completely different molds, potentially resulting in an increase in the number of manufacturing steps and mold cost. Moreover, a mold for forming the resistor portion on which the recess is to be formed must be combined with a core for forming the recess which can move toward and away from the mold; therefore, the mold is likely to become expensive.
(2) The conventionally-configured ceramic resistor generates heat such that temperature is high at a front end part of the first resistor portion and drops rearward along the axial direction. Thus, a steep temperature gradient is likely to be developed along the axial direction (the joining direction) between the first resistor portion, which generates a relatively high amount of heat, and the second resistor portion, which is at a relatively low temperature. In the ceramic heater disclosed in the above-mentioned publication, the cross-sectional ratio between the first resistor portion and the second resistor portion, which are formed from different kinds of ceramic, changes abruptly in a stepwise fashion at a joint where the protrusion and the recess are engaged. Therefore, when the above-mentioned temperature gradient arises, the effect of alleviating thermal stress concentration at the joint cannot be expected to be strong.
A first object of the present invention is to provide a ceramic heater that can be manufactured at low cost. The ceramic heater has a ceramic resistor in the form of a joined body consisting of different kinds of resistor portions. A second object of the present invention is to provide a ceramic heater in which a joint portion between different kinds of resistor portions exhibits excellent strength and durability. The present invention also provides a glow plug using such a ceramic heater.
A ceramic heater of the present invention includes a rodlike heater body which is configured such that a ceramic resistor formed of an electrically conductive ceramic is embedded in a ceramic substrate formed of an insulating ceramic. The heater body is also configured such that the ceramic resistor comprises a first resistor portion, which is disposed at a front end portion of the heater body and formed of a first electrically conductive ceramic, and a pair of second resistor portions, which are disposed on the rear side of the first resistor portion in such a manner as to extend along the direction of the axis of the heater body, whose front end parts are joined to corresponding end parts of the first resistor portion as viewed along the direction of electricity supply. The second resistor portions are formed of a second electrically conductive ceramic having an electrical resistivity that is lower than that of the first electrically conductive ceramic. The ceramic resistor assumes the form of a joined body consisting of resistor portions of different resistivities, for a reason similar to that described previously in relation to the conventional ceramic heaters.
To achieve the above-described first object, a first configuration of a ceramic heater according to the present invention includes at least a portion of a joint interface between the first resistor portion and the second resistor portion being deviated from a plane perpendicularly intersecting the axis of the heater body, and the joint interface is formed of a plane, a curved surface, or a combination thereof perpendicularly intersecting a reference plane defined as a plane including the axis of the heater body and the axis of the second resistor portion.
Since at least a portion of the joint interface between the resistor portions deviates from a plane perpendicularly intersecting the axis of the heater body, the area of the joint is increased as compared with the case where the joint interface assumes a simple plane perpendicularly intersecting the axis of the heater body, thereby enhancing the joining strength of the two resistor portions. With a plane including the axes of the second resistor portions being defined as a reference plane, the joint interface is formed of a plane, a curved surface, or a combination thereof perpendicularly intersecting the reference plane, thereby yielding the following advantage. When the ceramic resistor is manufactured through injection molding; specifically, by an insert molding process in which a green body of one resistor portion serves as an insert, and the other resistor portion is integrated with the insert through insert molding, mold sharing can be implemented, and the manufacturing process can be greatly simplified, thereby greatly reducing manufacturing cost.
The present invention provides a specific method for manufacturing a ceramic heater. The method comprises the steps of manufacturing a ceramic green body and firing the ceramic green body in order to manufacture the heater body. The ceramic green body comprising a green body, which is to become the ceramic substrate, and a resistor green body, which is embedded in the green body and is to become the ceramic resistor. In manufacture of the ceramic green body, the resistor green body is manufactured through injection molding, and in order to carry out the injection molding, a split mold having an injection cavity for molding the resistor green body is prepared. The split mold comprises a first mold and a second mold. The injection cavity is divided into a cavity formed in the first mold and a cavity formed in the second mold, along a dividing plane corresponding to the reference plane. The second mold has a second integral injection cavity formed therein. The second integral injection cavity integrally comprises a cavity corresponding to the first resistor portion, and a cavity corresponding to the second resistor portion. A preliminary-molding mold and an insert-molding mold are prepared to serve as a first mold. The preliminary-molding mold has a partial injection cavity formed therein for molding a preliminary green body, which is to become either the first resistor portion or the second resistor portion. The preliminary-molding mold comprises a filler portion for filling, when mated with the second mold, a portion of the second integral injection cavity which is not used for molding the preliminary green body. The filler portion has an adjacent face adjacent to the partial injection cavity and perpendicular to the dividing plane. The insert-molding mold has a first integral injection cavity formed therein. The first integral injection cavity integrally comprises a cavity corresponding to the first resistor portion, and a cavity corresponding to the second resistor portion.
The second mold and the preliminary-molding mold are mated with each other, and a molding compound is injected to thereby mold the preliminary green body. Next, the second mold and the insert-molding mold are mated with each other while the preliminary green body is disposed as an insert in the corresponding cavity portions of the first integral injection cavity and the second integral injection cavity, and a molding compound is injected into the remaining cavity portions to thereby yield the resistor green body through integration of an injection-molded portion with the preliminary green body.
The above-described method uses a split mold as an injection mold for forming a ceramic resistor as in the case of ordinary injection molding. The ceramic resistor; i.e., the first resistor portion and the two second resistor portions extending in the same direction from the corresponding ends of the first resistor portion and serving as electricity conduction paths, assumes a shape peculiar to a ceramic heater to which the present invention is applied, such as a shape resembling the letter U or a shape resembling the letter C. When the ceramic resistor (resistor green body) in such a form is to be formed through injection molding, a plane including the respective axes of the two second resistor portions is defined as a reference plane and is used as a dividing plane for dividing an injection cavity formed in a mold, thereby facilitating the removal of an injection-molded body from the mold.
The method of the present invention employs an insert molding process in which either the first resistor portion or the second resistor portion is formed beforehand as a preliminary green body, and the preliminary green body is integrated with the other resistor portion(s) through insert molding. A single second mold and two first molds are prepared to form a split mold for use in the insert molding. The second mold has a second integral injection cavity formed therein. The second integral injection cavity integrally comprises a cavity corresponding to the first resistor portion, and a cavity corresponding to the second resistor portion. The second mold is used in common in forming the preliminary green body and insert molding. The two first molds are a preliminary-molding mold for forming a preliminary green body and a regular mold for use in insert molding. The preliminary-molding mold has a partial injection cavity formed therein for molding the preliminary green body and comprises a filler portion for filling a portion of the second integral injection cavity which is not used for molding the preliminary green body, whereby the preliminary green body can be rationally formed merely by using a necessary portion of the second integral injection cavity. The joint interface between the first resistor portion and the second resistor portion assumes the form of a plane, a curved surface, or a combination thereof perpendicularly intersecting the above-mentioned reference plane; i.e., the dividing plane for dividing an injection mold cavity, whereby the mold can be readily opened without inflicting damage to the preliminary green body, by separating the preliminary-molding mold from the second mold in a direction perpendicular to the above-mentioned dividing plane. That is, as a result of the above-described shape of the joint interface, an end face of the preliminary green body which is to become the joint interface; i.e., the contact face between the preliminary green body and the filler portion (i.e., an adjacent face adjacent to the filler portion and the partial injection cavity), becomes parallel with the mold opening direction, thereby avoiding interference between the locus of the moving filler portion and the preliminary green body in the course of the mold opening.
After mold opening, while the preliminary green body is left in the same second mold, the first mold is replaced with the regular mold, followed by insert molding to thereby integrally mold the remaining portion. In this manner, the resistor green body can be readily obtained, and the second mold can be used in common for preliminary molding and regular molding (insert molding) to thereby reduce mold cost. That is, while assuming the form of a joined body consisting of resistor portions of different kinds, the ceramic resistor can be manufactured at low cost, thereby achieving the first object of the present invention.
To achieve the above-described second object, a second configuration of a ceramic heater according to the present invention has the joint interface between the first resistor portion and the second resistor portion mainly (specifically, not less than 50% of the joint interface) formed of an inclined face portion, which is inclined with respect to a plane perpendicularly intersecting the axis of the heater body.
Since the joint interface between the first resistor portion and the second resistor portion includes the above-described inclined face portion, the area of the joint is increased as compared with the case where the joint interface assumes a simple plane perpendicularly intersecting the axis of the heater body, thereby enhancing the joining strength of the two resistor portions. Also, the inclined face portion is simple in shape as compared with, for example, a protrusion-recess-fitting face, thereby reducing mold cost in forming the resistor portions by injection molding or a similar process. Since the joint interface assumes a simple shape, for example, when either the first resistor portion or the second resistor portion is formed beforehand as a preliminary green body, and the preliminary green body is integrated with the other resistor portion(s) through insert molding, a molding compound is favorably distributed along the joint interface. As a result, the joint interface becomes unlikely to suffer a defect, such as remaining bubbles.
Since, at the inclined face portion, the distribution ratio between a ceramic of the first resistor portion and that of the second resistor portion changes gradually along the axial direction of the heater body, even when a great temperature gradient arises along the axial direction, a joint portion is unlikely to suffer thermal stress concentration. Therefore, even when the heater is subjected to repeated thermal shock or a like condition, the joint portion can maintain good durability. In this manner, the second object is achieved.
Preferably, to enhance the above-described effect, the joint interface between the first resistor portion and the second resistor portion is entirely formed of the inclined face portion. However, in this case, for example, in manufacture of the ceramic resistor by the aforementioned insert molding process, an end face of the preliminary green body which is to become the joint interface includes a sharp end portion. As a result, chipping or a similar problem is likely to occur. In order to prevent the problem, an end portion of the joint interface may assume the form of a gently inclined face or a face perpendicularly intersecting the axis of the heater body.
The above-described first configuration and second configuration of a ceramic heater of the present invention may be combined with each other. In this case, the aforementioned first and second objects can be simultaneously achieved.
A glow plug of the present invention includes the above-described ceramic heater of the present invention; a metallic sleeve disposed in such a manner as to circumferentially surround the heater body of the ceramic heater and such that a front end portion of the heater body projects therefrom along the direction of the axis; and a metallic shell joined to a rear end portion of the metallic sleeve as viewed along the direction of the axis and having a mounting portion formed on an outer circumferential surface thereof, the mounting portion being adapted to mount the glow plug onto an internal combustion engine. Employment of the ceramic heater of the present invention can realize a glow plug exhibiting excellent durability at low cost.
In the claims appended hereto, the reference numerals associated with the recited components are cited from the accompanying drawings for a fuller understanding of the nature of the present invention, but they should not be construed as limiting the concept of the components in the claims.
The above and other features of the invention including various and novel details of construction, combination of parts, and method steps will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular ceramic heater, glow plug, and fabrication method embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention.